The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from shot peening, roll burnishing, thermal-spray coating, or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, and coating condition. Each of these also includes detection of electromagnetic property changes associated with single or multiple cracks. Spatially periodic field eddy-current sensors have been used to measure foil thickness, characterize coatings, and measure porosity, as well as to measure property profiles as a function of depth into a part, as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
For the inspection of structural members in an aircraft, power plant, etc., it is desirable to detect and monitor material damage, crack initiation and crack growth due to fatigue, creep, stress corrosion cracking, etc. in the earliest stages possible in order to verify the integrity of the structure. This is particularly critical for aging aircraft, where military and commercial aircraft are being flown well beyond their original design lives. This requires increased inspection, maintenance, and repair of aircraft components, which also leads to escalating costs. For example, the useful life of the current inventory of aircraft in the U.S. Air Force (e.g., T-38, F-16, C-130E/H, A-10, AC/RC/KC-135, U-2, E-3, B-1B, B-52H) is being extended an additional 25 years at least [Air Force Association, 1997, Committee, 1997]. Similar inspection capability requirements also apply to the lifetime extension of engine components [Goldfine, 1998].
Safely supporting life extension for structures requires both rapid and cost effective inspection capabilities. The necessary inspection capabilities include rapid mapping of fatigue damage and hidden corrosion over wide areas, reduced requirements for calibration and field standards, monitoring of difficult-to-access locations without disassembly, continuous on-line monitoring for crack initiation and growth, detection of cracks beneath multiple layers of material (e.g., second layer crack detection), and earlier detection of cracks beneath fastener heads with fewer false alarms. In general, each inspection capability requires a different sensor configuration.
The use of eddy-current sensors for inspection of critical locations is an integral component of the damage tolerance and retirement for cause methods used for commercial and military aircraft. The acceptance and successful implementation of these methods over the last three decades has enabled life extension and safer operation for numerous aircraft. The corresponding accumulation of fatigue damage in critical structural members of these aging aircraft, however, is an increasingly complex and continuing high priority problem. Many components that were originally designed to last the design life of the aircraft without experiencing cracking (i.e., safe life components) are now failing in service, both because aircraft remain in service beyond original design life and, for military aircraft, because expanded mission requirements expose structures to unanticipated loading scenarios. New life extension programs and recommended repair and replacement activities are often excessively burdensome because of limitations in technology available today for fatigue detection and assessment. Managers of the Aircraft Structural Integrity Program (ASIP) are often faced with difficult decisions to either replace components on a fleet-wide basis or introduce costly inspection programs.
Furthermore, there is growing evidence that (1) multiple site damage or multiple element damage may compromise fail safety in older aircraft, and (2) significant fatigue damage, with subsequent formation of cracks, may occur at locations not considered critical in original fatigue evaluations. In application of damage tolerance, inspection schedules are often overly conservative because of limitations in fatigue detection capability for early stage damage. Even so, limited inspection reliability has led to numerous commercial and military component failures.
A better understanding of crack initiation and short crack growth behavior also affects both the formulation of damage tolerance methodologies and design modifications on new aircraft and aging aircraft. For safe-life components, designed to last the life of the aircraft, no inspection requirements are typically planned for the first design life. Life extension programs have introduced requirements to inspect these xe2x80x9csafe-lifexe2x80x9d components in service since they are now operating beyond the original design life. However, there are also numerous examples of components originally designed on a safe-life basis that have failed prior to or near their originally specified design life on both military and commercial aircraft.
For safe-life components that must now be managed by damage tolerance methods, periodic inspections are generally far more costly than for components originally designed with planned inspections. Often the highest cost is associated with disassembly and surface preparation. Additionally, readiness of the fleet is directly limited by time out of service and reduced mission envelopes as aircraft age and inspection requirements become more burdensome. Furthermore, the later an inspection uncovers fatigue damage the more costly and extensive the repair, or the more likely replacement is required. Thus, inspection of these locations without disassembly and surface preparation is of significant advantage; also, the capability to detect fatigue damage at early stages can provide alternatives for component repair (such as minimal material removal and shotpeening) that will permit life extension at a lower cost than current practice.
In general, fatigue damage in metals progresses through distinct stages. These stages can be characterized as follows [S. Suresh, 1998]: (1) substructural and microstructural changes which cause nucleation of permanent damage, (2) creation of microscopic cracks, (3) growth and coalescence of these microscopic flaws to form xe2x80x98dominantxe2x80x99 cracks, (4) stable propagation of the dominant macrocrack, and (5) structural instability or complete fracture.
Although there are differences of opinion within the fatigue analysis community, Suresh defines the third stage as the demarcation between crack initiation and propagation. Thus, the first two of the above stages and at least the initial phase of Stage 3 are generally thought of, from a practical engineering perspective, as the crack initiation phase.
In Stage 1, microplastic strains develop at the surface even at nominal stresses in the elastic range. Plastic deformation is associated with movement of linear defects known as dislocations. In a given load cycle, a microscopic step can form at the surface as a result of localized slip forming a xe2x80x9cslip linexe2x80x9d. These slip lines appear as parallel lines or bands commonly called xe2x80x9cpersistent slip bandsxe2x80x9d (PSBs). Slip band intrusions become stress concentration sites where microcracks can develop.
Historically, X-ray diffraction and electrical resistivity are among the few nondestructive methods that have been explored for detection of fatigue damage in the initiation stages. X-ray diffraction methods for detection of fatigue damage prior to microcracking have been investigated since the 1930""s [Regler, 1937; Regler, 1939]. In these tests, fatigue damage was found to be related to diffraction line broadening. More recently Taira [1966], Kramer [1974] and Weiss and Oshida [1984] have further developed the X-ray diffraction method. They proposed a self-referencing system for characterization of damage, namely the ratio of dislocation densities as measured 150 micrometers below the surface to that measured 10-50 micrometers below the surface. The data obtained to date suggest that in high strength aluminum alloys the probability of fatigue failure is zero for dislocation density ratios of 0.6 or below. However, it is generally impractical to make such measurements in the field.
Electrical resistivity also provides a potential indication of cumulative fatigue damage. This is supported by theory, since an increase in dislocation density results in an increase in electrical resistivity. Estimates suggest that, in the case of aluminum, depending on the increase in the density of dislocations in the fatigue-damage zone, the resistivity in the fatigue-affected region may increase by up to 1% prior to formation of microcracks. These estimates are based on dislocation densities in the fatigue-damage zone up to between 2(1011 cmxe2x88x922 to 1012 cmxe2x88x922 and a resistivity factor of 3.3(10xe2x88x9219((cm 3 [Friedel, 1964].
Aspects of the inventions described herein involve novel inductive sensors for the measurement of the near surface properties of conducting and magnetic materials. These sensors use novel winding geometries that promote accurate modeling of the response, eliminate many of the undesired behavior in the response of the sensing elements in existing sensors, provide increased depth of sensitivity by eliminating the coupling of spatial magnetic field modes that do not penetrate deep into the material under test (MUT), and provide enhanced sensitivity for crack detection, localization, crack orientation, and length characterization. The focus is specifically on material characterization and also the detection and monitoring of precrack fatigue damage, as well as detection and monitoring of cracks, and other material degradation from testing or service exposure.
Methods are described for determining anisotropic material property variations with spatially periodic field eddy current sensors. Sensors with extended portions for forming the magnetic field, when placed in proximity to a MUT, provide a directionally dependent measure of the electromagnetic properties of the MUT, such as the electrical conductivity or magnetic permeability. Measurements of the material properties with varying orientations of the sensor with respect to the material property variation directions then provides a technique for determining the properties of interest.
For example, when measurements of the electrical conductivity are made in orthogonal directions on roller burnished metals such as aluminum, the anisotropy associated with the cold working and the quality of the process can be determined. Similarly, measurements of the magnetic permeability in orthogonal directions can be used to ascertain the level of fatigue damage, thermal damage, and stress level. The orientation and value of the maximum and minimum electrical property values can be determined from a continuous rotation of a spatially periodic field eddy-current sensor and can be used to determine the direction and magnitude of an applied load or the orientation and depth of a crack. Multiple frequency excitations can be used to vary the effective depth of penetration of the magnetic field into the test material, thereby extending the capability to measure electrical property variations with depth from the surface.
Novel inspection methods are described herein that relate to new surface mountable eddy-current sensors. These thin, conformable sensors can be mounted on conducting or magnetic materials, metals for example, and used to monitor crack initiation and growth by periodically measuring the sensor response to the material properties beneath the sensor footprint. Spatially periodic field eddy-current sensors, such as the Meandering Winding Magnetometer (MWM), can provide absolute properties measurements using single sensing elements or arrays of sensing elements (MWM-Arrays) located throughout the primary winding meanders. In one embodiment, one of the sensing elements in an array of sensing elements can be located in a place of minimal damage and can be used a reference for the measurement. The change in the electrical property exceeding the background noise level can also be used as an indicator for substantial changes in the MUT condition. In another embodiment, one of the elements can be left in xe2x80x9cairxe2x80x9d so that an xe2x80x9cairxe2x80x9d measurement can be used as the reference. In another embodiment of this method, the temperature of the MUT can be varied as a method of verifying the response of the individual sensing elements. The sensing elements can also be calibrated from the measured responses at several temperature levels and knowledge of the temperature dependence of the MUT properties. Alternatively, the use of a temperature sensor can allow the measurements to be compensated for temperature variations. While windings are typically used as the sensing elements, to provide an absolute or differential measurement of the material properties, other embodiments include the use of magnetoresistive elements and SQUIDS.
In another embodiment, the capability of a spatially periodic field eddy current sensor to provide reliable measurements with a sufficiently large gap (stand off) between the surfaces also provides the capability to monitor initiation of stress corrosion cracking and propagation of stress corrosion cracks. The MWM sensor or MWM-Arrays should not modify the environment that is causing the stress corrosion cracking and, for example, do not create tight crevices between the monitored surface and the MWM. Additionally, in one embodiment the substrate of the MWM sensor perforated between the windings further minimizes any differences between the bulk environment and the environment in the gap between the MWM and the measured surface. Also, in another embodiment a deep penetration surface mounted MWM or MWM-Array can be mounted on the inside skin to monitor hidden corrosion or cracking on the opposite side of the skin. For example, second or third layer corrosion and cracking can be monitored with an MWM-Array mounted on the inside of the third layer. Guides or conduits can be used to permit scanning eddy-current sensors to inspect through the permanently mounted eddy-current sensor. Furthermore, remote actuators can be used to vary the location of the drive and sense windings relative to the MUT.
When mounted to the test article, the sensor can be flexible to conform to the shape of the surface of the MUT. The sensor may also be mounted in difficult-to-access locations where only the connection leads or cables for connecting to the sensor leads are readily accessible. An example would be the mounting of a sensor between layers of materials or beneath fasteners such as those used in aircraft. Sealant materials can provide mechanical support and environmental protection for the sensor. In addition, electronics components can be placed near the sensing elements to provide signal amplification from the sensor. The mounted electromagnetic sensor may be a dielectrometer. Additional sensors can also be used in conjunction with the mounted electromagnetic sensor, such as a strain gauge or a temperature gauge.
Methods for characterizing MUT property variations over wide areas. Scanning of spatially periodic field eddy-current sensors over a surface permits the creation of images of the absolute material properties beneath the sensor. Patterns in the images of the properties, such as the electrical conductivity, can indicate locations of damage. Furthermore, multiple frequency or multiple orientation measurements can be used to characterize the damage, such as distinguishing cracks from fretting damage, determining crack size or morphology, detecting hidden corrosion or cracks, determining properties of coating layers, and differentiating damage from manufacturing conditions. The capability to create images of damage allows the extent of the damage to be identified so that the size and type of patches can be determined. For areas of wide-spread or distributed fatigue damage, identifying locations of damage can be used to schedule future inspections or repair and replacement actions. In one embodiment, the flexible sensor can be mounted to a compliant layer that is affixed to a substrate that approximates the shape of the surface of the MUT. In another embodiment, two or more sensing elements can be situated to pass over the same flaw with a single scan. The signals from the individual sensing elements can then be processed and combined to enhance the flaw signal.
In one embodiment, a method for measuring properties of interest co periodic field eddy current sensor having extended portions; passing a time varying electric current through the extended portions to form a magnetic field; placing the sensor in proximity to a test material; measuring an electrical property of the test material with plural orientations of the extended portions relative to the test material; and the electrical property measurements coinciding with two or more directions relative to the extended portions to at least one property of interest. The two electrical property measurements may be made in orthogonal directions.
The two measurements may be orthogonal components of electrical conductivity. The properties of interest may be rolling direction and roller burnishing quality, and the ratio of the electrical conductivities may be related to the properties of interest. The electrical property may be magnetic permeability. The property of interest may also be the level of fatigue damage.
The properties of interest may be applied or residual stress direction and level. The property of interest may also be a thermal aging level.